Xylose fermentation

Our standard sugar mixture (Std. Mix) for fermentation contained YEP, and 70 g/L of pure glucose and 40 g/L of pure D-xylose. Glucose in Std. Mix was completely fermented in 10 h. Higher rates of xylose fermentation started when glucose concentration dropped below 2.5%. Of the xylose 88.5% was fermented in 30 h (Fig. 1). The production of xylitol dur-... 

Zadivar et al. (12) reported fermentation of a pure glucose/xylose mixture using their xylose-fermenting S. cerevisiae, but xylose mainly converted to xylitol with ethanol only as a minor product (12). Furthermore, the time needed for completion of the fermentation exceeded 100 h with an initial xylose concentration of 50 g/L. 

The fifth paper, "A Separative Bioreactor Direct Product Capture and pH Control," presented by Seth Snyder of the Argonne National Laboratory, reviewed development and performance of a novel bioreactor incorporating electrodeionization to simultaneously produce and separate products from both enzymatic and microbially mediated reactions. The sixth paper, " Optimization of Xylose Fermentation in Spent Sulfite Liquor by Saccharomyces cerevisiae 259ST," presented by Steven Helle of the University of British Columbia, provided an overview of an approach to fermentation optimization utilized to identify key process variables limiting use of the SSL for commercial ethanol production. 

It is already known that under anaerobic conditions the xylose-fermenting yeasts do not produce ethanol because the electron transport in the respiratory chain is not activated (20). Since either aeration or oxidizing compounds, such as acetone, could increase the ethanol yield, the effect of improving the oxygenation of the medium or adding oxidizers was investigated using an STR partially filled with marbles. 

Performance of Some Recombinant and Native Xylose-Fermenting Organisms Under Laboratory Conditions and on Native Lignocellulosic Hydrolysates... 

E. Lohmeier-Vogel, K. Skoog, H. Vogel and B. Hahn-Hagerdal (1989). 31P Nuclear magnetic resonance study of the effect of azide on xylose fermentation by Candida tropicalis. Appl. Envinron. Microbiol., 55, 1974-1980. 

Forest Products Laboratory have discovered a xylose-fermenting yeast (Candida tropicalis). Thus it now is possible to convert all wood sugars to ethyl alcohol with a combination of yeasts. Isolation of the specific enzymes and genetic engineering of the enzymes could dramatically improve the efficiency of this conversion. 

Helle, S., Cameron, D., et al., Effect of inhibitory compounds found in biomass hydrolysates on growth and xylose fermentation by a genetically engineered strain of Saccharomyces cerevisiae. Enzyme Microbial Technology 2003, 33 (6), 786-792. 

Zhang, M., Franden, M. A., Newman, M., McMillan, J., Finkelstein, M., and Picataggio, S., Promising ethanolo-gens for xylose fermentation—Scientific note. Applied Biochemistry Biotechnology 1995, 51-2, 527-536. 

Kuyper, M., Hartog, M. M., Toirkens, M. J., Almering, M. J., Winkler, A. A., van Dijken, J. P., and Pronk, J. T., Metabolic engineering of a xylose-isomerase-expressing Saccharomyces cerevisiae strain for rapid anaerobic xylose fermentation. FEMS Yeast Res 2005, 5 (4-5), 399-409. 

Jeffries, T. W., Dahn, K., Kenealy, W. R., Pittman, P., Sreenath, H. K., and Davis, B. P., Genetic-engineering of the xylose-fermenting yeast Pichia-Stipitis for improved ethanol-production. Abstracts of Papers of the American Chemical Society 1994, 207, 167-BTEC. 

This intractable problem may now be close to being solved. A Saccharomyces species that expressed the xylose isomerase gene from an anaerobic fungus was found to grow slowly on pentoses [29]. Improvement resulted from a combination of rational engineering - overexpression of the pentose phosphate-converting enzymes (see Fig. 8.5) - and classical strain improvement [30]. The authors conclude The kinetics of xylose fermentation are no longer a bottleneck in the industrial production of ethanol with yeast ... 

Genetic Engineering for Improved Xylose Fermentation by Yeasts... 

Keywords. Xylose, Fermentation, Respiration, Metabolic engineering. Regulation, Regulatory mechanisms. Pentose metabolism. Oxygen regulation. Glucose regulation... 

Initially, the basis for xylose fermentation in yeasts and fungi was not well understood. Researchers knew that bacteria with xylose isomerase could ferment xylose while fungi with the oxidoreductase uptake system could not, so they sought to express xylose isomerase in S. cerevisiae or other yeasts in order to create an improved xylose fermenter. Ueng et al. [34] cloned the gene for xylose isomerase from E. coli and Chan et al. [35] expressed it in S. pombe. These are the only researchers to have reported success with this approach. Amore et al. [36] expressed the genes from Bacillus and Actinoplanes in S. cerevisiae. Approximately 5 % of the cellular protein consisted of xylose isomerase, but it was not catalytically active. Sarthy et al. [37] expressed E. coli xylose isomerase in S. cerevisiae but found that the protein had only about 10 as much activity as the native protein from E. coli. 

Metabolic engineering [39, 40] has been used to impart the capacity for ethanol production and xylose fermentation in E. coli [41-45], Klebsiella oxytoca [A6,A7],Zymomonas mobilis [48,49] andS. cerevisiae [50-53]. In general, attempts at metabolic engineering have been more successful in bacteria than in yeasts. Although the reasons are not entirely clear, the smaller genomes and fewer feedback regulatory factors found in bacteria make these organisms much easier to work with. 

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